The present invention relates to a heat spreader that can diffuse heat, for example, even when the quantity of heat increases due to high integration design of semiconductor devices, a semiconductor device provided with the heat spreader, and a method of making the same.
In the CPU (central processing unit) of a computer, workstation, personal computer (PC), and etc., a ceramic package called a PGA (pin grid array) is mainly used, and heat generated from a silicon chip is diffused via a heat-release substrate (heat spreader) between the silicon chip and a heat sink fin made of Al. In recent LSIs, it becomes very important to dissipate the heat generated from silicon chips in view of the high speed design and large power consumption design thereof, and especially in LSIs for microcomputers and logic ASICs (application specific ICs), the diffusion of heat is accelerated by bringing a heat spreader into contact with a silicon chip.
For example, a PGA (pin grid array) package shown in FIG. 8 comprises a heat spreader (11), a silicon chip (8), bonding wires (9), a ceramic substrate (10), pins (12), silver solder (13), and a lid (14). In this structure, the heat spreader (11) is in contact with the silicon chip (8) and, as a matter of course, it is important that the heat spreader (11) have good heat-diffusibility to release the heat generated from the silicon chip (8), and it is also of importance for the heat spreader (11) to have a thermal expansion coefficient similar to that of the silicon chip (8). Furthermore, because the heat spreader (11) is directly silver-brazed to the ceramic substrate (10), it is important that the heat spreader (11) be similar to the ceramic substrate (10) in the thermal expansion coefficient.
It is expected that demand for packages of this type will increase in the future. Also, it is necessary that heat spreaders for such applications, which are in contact with silicon chips, have thermal expansion matched to that of silicon chips and it is generally considered desirable that heat spreaders have 4 to 11.times.10.sup.-6 /.degree. C. or so as the average thermal expansion coefficient at 30-150.degree. C.
Conventionally, 0.5 to 1 mm thick sheets of about 30 mm square made of a Cu--W-base alloy or Mo, which meet these requirements, have been used as the heat spreaders for semiconductor devices. However, these materials are expensive and besides the weight of packages of these materials is heavy because of high specific gravity, which is 17.0.times.13.sup.3 kg/m.sup.3 for 89W-11Cu and 10.2.times.10.sup.3 kg/m.sup.3 for Mo. This is a great disadvantage in terms of downsizing design and weight reduction design, which are the trends in recent LSIs.
In packages of LSIs of a type in which conventional lead frames are used unlike the above PGA type LSIs, the lead frame itself is formed of copper and a copper alloy which diffuse heat well. In this case, however, because the thermal expansion coefficient of the lead frame is larger than that of the silicon chip, internal stresses in the interface between the silicon chip and the lead frame cause such a fear as cracks are formed in the silicon chip due to the generation of stresses during a production process or in use. In order to solve this problem, the present inventors and others proposed, in JP-A-8-232049, an invention relating to a composite material for electronic parts in which a sinter layer of powder mainly composed of copper or a copper alloy is formed at least on one surface of an Fe--Ni-base alloy sheet of low thermal expansion and a method of making this composite material.
Further, the combinations of a low-thermal-expansion material provided with through holes and a high-thermal-conductivity material have been proposed in JP-A-2-231751, JP-A-7-80272, and etc.
In packages of PGA, etc. in which no lead frame is used, however, thermal conduction in the thickness direction (, i.e., in the lamination direction) is inferior in a case of a structure in which copper and an Fe--Ni-base alloy layers are merely laminated and, therefore, this structure cannot be used as a heat spreader. In view of the foregoing, a new heat spreader has become necessary in place of Cu--W and Mo sheets, which new heat spreader is inexpensive and, at the same time, small and thin, and which new heat spreader permits a weight reduction. Incidentally, in packages of a type in which no lead frame is used, the above PGA, BGA (ball grid array) and CSP (chip size package) have been put into practical use and future large demand is expected.